Let's start by splitting up a circle (cake). Here we've cut it in half so there are two pieces:
Each piece of cake is exactly half of the original. We write this in numbers as ½.
The number on the bottom (denominator) tells us how many pieces we've split something up into.
The top number (numerator) tells us how many pieces we have.
If the top and bottom are the same, we have a whole cake (1)!